Shaped articles of aluminum nitride, especially fibers thereof, are sought because they possess high thermal conductivity; hence, they are useful in electronic applications where heat dissipation is necessary. Thermal conductivity may be an order of magnitude higher than that of alumina but depends critically on material purity. These considerations are discussed in U.S. Pat. No. 4,578,365, in relation to the preparation of bulk ceramic articles of aluminum nitride.
Fibers composed nominally of aluminum nitride may be prepared by a number of prior art methods. In one reaction utilizing inexpensive raw materials, alumina and carbon, or an organic precursor thereof, are heated at elevated temperatures in the presence of nitrogen, whereupon aluminum nitride is formed. This is a classic text-book reaction and is described in detail in E.P. 0213 629 and in U.S. Pat. No. 4,857,246. However, this reaction requires careful stoichiometric control and often results in excess carbon or unconverted alumina, either of which impacts dramatically on thermal conductivity. In fiber formation, incomplete sintering (densification) or excessive grain growth can also result, either of which results in inferior fibers.
Laubengayer et al., J. Amer. Chem. Soc., 83, pages 542 to 546 (1961), disclose the reactions of amines at low temperatures with aluminum alkyls or alkyl aluminum chlorides to form 1:1 addition compounds. When such addition compounds having N--H and Al--R bonding are pyrolyzed, aluminum nitride is produced.
Interrante et al., in Mat. Res. Soc. Symp. Proc. 73 (1986), pages 359-366, disclose the conversion of C.sub.2 H.sub.5 AlNH to aluminum nitride in the presence of ammonia. The aluminum nitride produced with ammonia contained less carbon than that formed without it. Interrante et al., also disclose that C.sub.2 H.sub.5 AlNH is converted to aluminum nitride at 300.degree. C. to 900.degree. C. with retention of morphology.
Japanese Patent 54-13439 discloses a method for the production of aluminum nitride in the form of a powder.
Additional relevant background references include the following: Strength of Aluminum Nitride Whiskers, Gribkov et al., Izvestiya Akademii Nauk SSSR, Neorganicheskie Materialy, Vol. 13 (10) pages 1775 to 1778, (1977); Role of Liquid Drops in the Growth of Filamentary Crystals of Aluminum Nitride, Portnoi et al., Izvestiya Akademii Nauk SSSR, Neorganischeskie Materialy, Vol. 6 (10) pages 1762 to 1767 (1970); and Growth of AlN Whiskers During the Nitriding of Aluminum, Portnoi et al., Poroshkovaya Metallurgiya No. 5 (89) pages 10 to 14 (1970). The methods of these additional references form whiskers, essentially acicular single crystal fibers. Thus, these are not polycrystalline AlN fibers as disclosed herein.
Canada 839,321 discloses AlN by carbothermal reduction of alumina. Japan 61-124626 discloses a method for making AlN fibers from aluminum metal fibers or from a solution of an aluminum compound. U.S. Pat. No. 3,846,527 and U.S. Pat. No. 4,010,233 make reference to metal nitride fibers. This route to AlN fibers' uses the carbothermal reductive nitridation reaction. Significant carbon residues characterize the AlN fibers disclosed in U.S. Pat. No. 4,740,574. U.S. Pat. No. 3,529,044 discloses aluminum carbide and that if nitrogen is used, then metal nitride fibers are produced. U.S. Pat. No. 3,658,979 discloses large diameter fibers with a thin film of AlN on the surface. EPA 213,629 discloses aluminum nitride fibers prepared by heating precursor fibers, spun from a solution of aluminum oxychloride and polyvinyl alcohol, in nitrogen. U.S. Pat. No. 4,761,388 discloses an inorganic fiber composed essentially of fine crystals of an aluminum compound, which contains at least 10% by weight of aluminum nitride and the rest of the aluminum compound being alumina.
A predominant theme among many of these references is to minimize the amount of oxygen in formed AlN fibers, to enhance thermal conductivity. This emphasis does not recognize the potential usefulness of AlN fibers as high strength reinforcement in composites and shaped articles. It would be valuable to develop an aluminum nitride fiber that exhibits superior strength and properties for these and other uses.
Accordingly, an object of the present invention is the development of aluminum nitride fibers having high strength. It is a further object of the present invention to provide high strength AlN fibers that are used to reinforce a multitude of shaped articles including composites. A feature of the present invention is the incorporation of significant amounts of oxygen into the fibers, to facilitate the high strength properties. It is an advantage of the present invention to provide AlN fibers that do not tend to adhere to one another.
These and other objects, features and advantages of the present invention will become readily apparent upon having reference to the following description of the invention.